The present invention relates to microcapsule insecticide compositions which are stabilized against environmental degradation.
Pyrethroids, including both the naturally occurring compounds and their synthetically prepared analogs, are a well known class of contact insecticides. They have broad spectrum activity, that is, they are effective in controlling a variety of pests such as houseflies, mosquitoes, cockroaches, etc. They are not harmful to plants, food, animals and humans and are consequently environmentally safe (leaving no harmful residues).
Despite these highly favorable characteristics, pyrethroids have had only limited usefulness because of their relatively short-lived insecticidal activity. This is due to their decomposition into non-active, non-insecticidal products in the presence of oxygen and ultraviolet light. The speed of this decomposition (or environmental degradation) is dependent upon the environment in which the pyrethroids are placed but typically takes place in from several minutes to several hours. Thus the usefulness of pyrethroids as insecticides has been severely limited by their instability.
Encapsulation of a variety of active liquid substances (including dyes, inks, chemical reagents, pharmaceuticals, flavoring materials, pesticides, herbicides and peroxides) has been suggested to preserve them until released by crushing, melting, dissolving or otherwise removing the capsule wall or until release by diffusion is effected (see for example U.S. Pat. No. 3,577,515). This encapsulation per se is not of great help in the delivery of pyrethroids, since they degrade almost as readily inside the capsules as they do unencapsulated.
Various specific attempts have been made to stabilize pyrethroids against environmental degradation. Thus, antioxidants, biological synergists and photostable ultraviolet light absorbent compounds have been added to solutions of the pyrethroids. Various solid carriers have also been used, such as gum arabic, dextrin, gelatin, unvulcanized rubbers, inorganic powders, and other polymeric products. These attempts have been at best only very moderately successful in reducing the degradation by atmospheric oxygen and ultraviolet radiation and extending the insecticidal life of pyrethroids. Furthermore, they leave behind unsightly residues which are difficult to remove. The pyrethroids are also easily dissolved and washed away by water rinsing (e.g. by rain) thereby preventing their timed release.